The present invention relates to gemstone polishing machines for working facets on a gemstone or the like. As used herein, the expression "gemstone or the like" includes natural or synthetic gemstones or semi-precious stones.
When working facets on gemstones, the greatest difficulties are encountered with diamonds because of their extreme hardness and also because of the effect of grain as discussed below. Thus, although the invention relates generally to gemstones, the detailed discussion of the invention is made with reference to diamonds. Generally, if a polishing machine can be used for diamonds, it can be used for any other gemstone, whether natural or synthetic.
Normal practice when working facets is to use a moving working member which contacts the stone in a working plane, and at the present time, the working member is normally in the form of a wheel or disc called a scaife, made of thick cast iron, of a diameter usually rather greater than 30 cm and usually rotating at 2,000-5,000 r.p.m., normally about 3,000 r.p.m. A gemstone polishing unit comprises means for holding and driving a moving working member and at least one polishing machine--there may for instance be four polishing machines spaced around a scaife.
The expression "working facets" is used to cover different but similar operations, namely grinding, blocking and polishing. Blocking is the removal of the bulk of the surplus material on the bottom (culet side) of a sawn stone, frequently prior to rebruting, with the intention of subsequently polishing all the bottom facets. In the art, a machine is called a polishing machine even if it is only used for grinding or blocking. Polishing is the formation of the finished facets on the top (table side) or bottom--there is for instance in GB Pat. No. 1 206 937 a good description of the finished facets on the bottom of a diamond.
When working the facets, the stone is held in a holder which normally is long-shaped with its axis substantially coincident with that of the stone. Thus, to work the facet, the holder will have its axis inclined to the working plane at an angle which determines the angle of the facet. Two movements are required of the holder, namely a movement about its own axis for changing the facet being worked (this is called changing the facet position), and movement about an axis normal to the working plane for changing the grain orientation. The phenomenon of grain in diamond working is well known, but is discussed for instance in GB Pat. No. 2 037 196A. It is necessary to present the facet to the scaife in such a way that the direction of movement of the scaife (tangential movement) makes an appropriate angle with a given crystallographic direction or grain direction in that facet.
Apart from this, the holder must be movable towards and away from the working plane, and one must determine its movement towards the working plane very accurately so as to know when the facet has been worked to the appropriate depth (accuracies of up to 2.5 um may be required for polishing diamond facets). When working has occurred to the correct depth, the stone must be raised from the working plane, the grain orientation and facet changed, and the stone gradually returned to the working plane with no or low pressure to avoid damage to the stone or to the working member. Subsequent to this, sufficient working pressure must be exerted to allow the working to proceed at an acceptable rate.
Working facets is often accompanied by a reciprocatory motion of the stone transversely across the working member, i.e. radially of a scaife, to even the wear of the working member and keep the facet smooth--this motion is called zooting herein.